1. Field of the Invention
The present invention relates to a back light unit. More particularly to a back light unit, in which the production of bright lines, derived from high-luminosity light condensing sections generated from point light sources is prevented, thereby allowing brightness to be uniformly maintained at a liquid crystal panel.
2. Description of the Related Art
A conventional back light unit functions as an illuminator for a non-emissive flat panel display such as a liquid crystal display, and can be divided into a direct light type and an edge light type according to arrangement of a light source.
The conventional direct light type back light unit is a back light unit in which the light source is installed just below the flat panel display, and thus, directly irradiates light to the flat panel display, whereas a conventional edge light type back light unit refers to a back light unit in which the light source is installed on one side of the flat panel display and thus, irradiates light to the flat panel display by means of a light guide plate that will be described below.
The light source is categorized into a line light source and a point light source. The typical line light source includes a cold cathode fluorescent lamp (CCFL) in which electrodes are installed to opposite ends of a tube, while the point light source includes a light emitting diode (LED).
A CCFL can emit a high intensity of white light, obtain high brightness and high uniformity, and provide a large-area layout. However, a CCFL is driven by a high-frequency alternating current signal and has a narrow operating temperature range.
In contrast, a LED is degraded in performance compared to a CCFL, but it is driven by a direct current signal, has a long lifetime and a wide operating temperature range, and can be made thin.
Meanwhile, the light guide plate is used in the edge light type back light unit, converts the incident light from the line or point light source into surface light, sends out the surface light in a vertical direction, and is made of acryl-based transparent resin having high light transmissivity such as poly methyl methacrylate (PMMA), or olefin-based transparent resin.
A reflector plate is provided on a rear surface of the light guide plate so as to return the light deflected from the light guide plate back to the light guide plate. An optical sheet is provided on a front surface of the light guide plate so as to diffuse the light emitted from the light guide plate.
Recently, a prism light guide plate having prisms formed on a rear surface thereof, has been used for high brightness and low costs. The prism light guide plate is a light guide plate having a prism pattern formed on the rear surface, at a fine pitch. Thus, the prism light guide plate can obtain brightness higher than the previous light guide plate, although one reverse prism sheet is used instead of a plurality of prism sheets.
As shown in FIG. 1, a conventional back light unit 10 includes a prism light guide plate 13, a plurality of point light sources 23 provided on the one side 18 of the prism light guide plate 13, and a reflector plate 27 provided below the prism light guide plate 13.
In this state, when the point light sources 23 emit light, the light is incident on an incoming light section 13a provided on one side 18 of the prism light guide plate 13, is reflected in the prism light guide plate 13, and then comes out of an outgoing light section 13b provided on an upper portion of the prism light guide plate 13.
Further, among the light incident on the prism light guide plate 13, the light deflected downwards is reflected by the reflector plate 27, is incident on the prism light guide plate 13 again, and then comes out through the outgoing light section 13b. 
However, when the point light sources 23 emit light toward the incoming light section 13a, the luminosity is relatively increased around the incoming light section as illustrated in FIG. 6A, so that dot-like light condensing sections are generated. The light of each light condensing section forms a spectrum by means of a prism pattern 16 of the prism light guide plate 13, so that bright lines are produced in an oblique direction, and thereby a quality of image is degraded.
In order to solve this problem, in the conventional back light unit each prism is rounded off, or the prism pattern is applied to the incoming light section. However, this method has its limits to remove the bright lines produced in an oblique direction.
The conventional edge light type back light unit is used in a laptop computer or a mobile phone. In the case of the mobile phone, the bright lines produced in an oblique direction make it impossible to use the prism light guide plate.